1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, or a facsimile, that forms an image by means of electrophotography.
2. Description of the Related Art
An image forming apparatus that forms an image by means of electrophotography includes a photosensitive drum serving as an image bearing member and a plurality of corona chargers arranged around the photosensitive drum. The plurality of corona chargers include a primary charger, a pre-transfer charger, a transfer charger, and a detach charger.
When activated, these corona chargers generate active substances (e.g., ozone and oxides of nitrogen). Some of these active substances change the chemical composition and the crystalline structure of the surface of the photosensitive drum. The change in the chemical composition and the crystalline structure of the surface may increase the hygroscopic property of the surface, and therefore, the electrical specific resistance of the surface area may decrease. Accordingly, the electrostatic charge retention properties of the drum may deteriorate, thus decreasing the quality of a formed image. In particular, when the photosensitive drum is left unused in a high-humidity environment for a long period of time due to being in a power-off mode or power-saving mode, moisture absorption of the photosensitive drum occurs at linear zones facing the corona chargers in a concentrated manner. Because a difference in the electrostatic charge retention function between the zones facing and not facing the corona chargers exists, uneven density or some defects may appear in the output image.
To solve this problem, it has been proposed that the photosensitive drum always be rotated to prevent the decrease in the electrostatic charge retention functions in particular zones. It has also been proposed that a heater be provided in a photosensitive drum and the heater is always in a power-on mode so as to uniformly heat the whole unoperated photosensitive drum. Thus, the photosensitive drum is prevented from absorbing moisture.
For example, a copying machine disclosed in Japanese Examined Utility Model Registration Application Publication No. 1-34205 includes a hollow photosensitive drum. Heated air is externally delivered to the hollow photosensitive drum so as to evenly heat the whole photosensitive drum.
A color printer disclosed in Japanese Patent Laid-Open No. 8-76641 includes a photosensitive drum having a roller-shaped heater on the outer periphery thereof. By rotating the photosensitive drum, the entire surface of the photosensitive drum can be evenly heated.
Copying machines disclosed in Japanese Patent Laid-Open No. 8-160821 and Japanese Patent Laid-Open No. 8-171337 include a heating element having an elongated plate shape at a position slightly spaced away from a photosensitive drum so as to heat the photosensitive drum across an air layer. At start-up time, the heating element enters a power-on mode so as to heat the air layer while rotating the photosensitive drum. Thus, the entire surface of the photosensitive drum is heated so as to eliminate the moisture.
When a resistance heating heater is provided in a photosensitive drum and is always in the power-on mode, power is consumed even during power-off time of a copying machine or printer and even out of hours. Also, an increase in cooling power at the installation location is required. This power consumption does not meet the increasing demand for power and energy conservation. Accordingly, Japanese Patent Laid-Open No. 8-160821 discloses technology in which an image forming apparatus stops the heating during power-off time or after a predetermined unoperated time period has elapsed (i.e., in a power-saving mode). When the image forming apparatus enters the power-on mode or exits the power-saving mode, the image forming apparatus starts heating prior to its printing operation so that a photosensitive drum is preheated for about 30 seconds to a couple of minutes to eliminate the moisture.
In the case of the preheating method discussed in Japanese Examined Utility Model Registration Application Publication No. 1-34205, the heat is dissipated together with the heated air, and therefore, heating efficiency is low. In addition, the temperature of the surface of the photosensitive drum does not rise rapidly, and therefore, a lengthy warm-up time is required for starting up the apparatus and starting up the printing process.
In the case of the preheating method discussed in Japanese Patent Laid-Open No. 8-76641, since the roller heater having a high temperature is in direct contact with the photosensitive drum, there is the possibility that toner will be heat-sealed on the surface of the photosensitive drum in the contact area or toner which inhibits heat conduction will be deposited onto the roller heater.
Additionally, in the case of the preheating method discussed in Japanese Patent Laid-Open No. 8-160821 and Japanese Patent Laid-Open No. 8-171337, the photosensitive drum is not in contact with the heater. Accordingly, the problem caused by the contact between the photosensitive drum and the heater does not occur. However, since the heat is transferred by an air layer, the heating efficiency is low. Thus, the temperature of the surface of the photosensitive drum rises slowly despite high power consumption of the heating element. If the power consumption of the heating element is increased to speed up the temperature rise, electric elements and components around the heating element are unnecessarily heated, and therefore, an additional cooling fan is required to cool an electronic circuit in the image forming apparatus.
The present inventor conducted an experiment in which the heating element was substituted by a halogen lamp heater, which was disposed at a position spaced slightly away from a photosensitive drum to eliminate the moisture by means of a radiant heating method. In this case, since the halogen lamp heater is not in contact with the photosensitive drum, the problems caused by the contact, such as a toner adhesion problem, do not occur. In addition, since the heat conduction does not rely on the air, high heating efficiency can be obtained. However, a typical halogen lamp heater equally disperses radiation throughout 360 degrees. Accordingly, in this experiment, parts and units adjacent to the halogen lamp heater (e.g., a cleaning unit and a developer unit) were unnecessarily heated, as will be described below. Thus, it was found that toner adhered to these parts and units.